A variety of devices employ power supply circuitry in order to provide voltages and/or currents used in operating the device itself or used in the operation of the device. Such power supply circuitry is often utilized to alter power supplied from a power source, such as line power or a battery, to meet the demands or requirements of the device. For example, power supply circuitry may be utilized to step-up/step-down current, step-up/step-down voltage, provide a direct current (DC) output from an alternating current (AC) input, and/or provide an AC output from a DC input. Circuitry providing the foregoing may be relatively complex, perhaps including a number of active components, and often suffers from inefficiencies, e.g., an appreciable amount of the power supply energy is consumed in altering the power supplied from the power source. However, complex and inefficient power supply circuits are undesirable in a number of situations, such as in certain portable devices using a battery as a power supply.
An implantable neurostimulator, such as the GENESIS™ Neurostimulation System available from Advanced Neuromodulation Systems, Plano Tex., is one example of a portable device which may implement power supply circuitry as described above in altering one or more aspects of a battery power supply output for use in operating the device or in the operation of the device. Because implanting and explanting such a neurostimulator causes appreciable trauma to the patient, it is typically desired that the neurostimulator power supply be small and relatively long lived and that the circuitry (including power supply circuitry) of the neurostimulator be small and reliable. Even where a rechargeable battery is used as a power supply, it is typically desired to provide operation of the neurostimulator in such a way as to result in a relatively long battery life between recharge cycles so as to minimize restrictions on the patient's mobility. Battery voltage associated with a battery such as may be implemented in a neurostimulator can be 2.5 volts, for example, or on the order of 4.2 volts, for lithium ion, but in any case is typically relatively low due to the size constraints of the device.
In providing neurostimulation, it may be desirable to provide up to 30 milliamp pulses, for example, to an area of the patient's anatomy, such as near the spinal cord. The patient equivalent resistance in the area of the delivery of the therapeutic current can often range from 200 ohms to 2 kilohms. Knowing the current and load, the voltage needed to effect the desired therapy can be calculated in the above example as being on the order of 15 volts to provide the desired current to the patient. However, as described above, the battery voltage may be much less than 15 volts.
Accordingly, the above mentioned neurostimulator may implement power supply circuitry which provides voltage up-conversion to facilitate delivery of therapy to the patient using the available power supply. There are many ways to implement voltage up-conversion. However, in a battery powered device, in particular, it is generally desirable to provide the voltage up-conversion in the most efficient way possible.
In the past a number of power supply circuit configurations have been implemented to provide voltage up-conversion. In particular, inductive voltage up-converters (voltage up-converters also being referred to herein as voltage multipliers) and capacitive voltage up-converters.
Inductive voltage up-converters or voltage multipliers require the use of a coil for voltage conversion, which in turn necessitates the use of alternating current. However, batteries such as those used in the above mentioned neurostimulators provide direct current. Accordingly, the use of an inductive voltage multiplier with a battery power supply generally involves the use of complicated and inefficient switching regulator circuitry to convert the direct current from the battery to alternating current for voltage up-conversion. Moreover, additional rectifier circuitry is typically implemented to convert the up-converted alternating current back to direct current. Electromagnetic noise is often introduced by inductor-based switching regulator circuitry, such as may interfere with data communications, thereby requiring additional shielding and/or circuitry to prevent such electromagnetic noise from interfering with operation of the device. Accordingly, inductive voltage up-converters are generally undesirable for use in small battery powered devices, such as implantable neurostimulators.
Capacitive voltage up-converters or voltage multipliers have generally been used to provide output voltages in integer multiples of the battery voltage (e.g., 2 times the battery voltage, 3 times the battery voltage, etcetera). However, such integer multiples of the battery voltage often are not the most efficient voltages. For example, assuming a battery voltage of 4 volts and that a desired current for therapeutic stimulation requires 9 volts, a typical prior art capacitive voltage multiplier must provide 12 volts for the needed 9 volt pulse because its design provides selection between 4 volts (1 times the 4 volt battery voltage), 8 volts (2 times the 4 volt battery voltage), 12 volts (3 times the 4 volt battery voltage), etcetera.
The power consumed from the battery is based on the multiplicative factor created from the battery. In the foregoing example, if 10 milliamps was delivered to the patient, 30 milliamps was pulled from the battery because of the 3 times multiplicative factor used in the voltage up-conversion. Accordingly, if the voltage provided by the power supply circuitry could be controlled to more closely match that needed for the desired level of stimulation, the multiplicative factor, and thus the power pulled from the battery, could be reduced.
Further compounding the inefficiencies associated with capacitive voltage multipliers of the prior art is their operation in creating and storing a voltage multiple. In operation, a capacitive voltage multiplier will create a particular multiply voltage and store that voltage on a storage capacitor for output by the power supply circuit. When it is desired to change the output voltage of the power supply circuit, the previously stored multiply voltage stored by the storage capacitor must be discharged in order to change the voltage to a new value. The discharge of the previously stored multiply voltage is a waste of energy which, if done often, can amount to an appreciable drain on the power supply. For example, where two sequential stimulation pulses of a neurostimulator require different voltages, prior art capacitive voltage multipliers would require discharging of capacitors charged for providing the first voltage for recharging of the capacitors for providing the second voltage, thereby wasting energy.
Accordingly, a need exists in the art for an efficient voltage multiplier, such as may be used in relatively small, battery powered devices.